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<h3 class="section">2.12 Targets</h3>


<p><strong>Description</strong><br>
Each port of BFD to a different machine requires the creation
of a target back end. All the back end provides to the root
part of BFD is a structure containing pointers to functions
which perform certain low level operations on files. BFD
translates the applications&rsquo;s requests through a pointer into
calls to the back end routines.
</p>
<p>When a file is opened with <code>bfd_openr</code>, its format and
target are unknown. BFD uses various mechanisms to determine
how to interpret the file. The operations performed are:
</p>
<ul>
<li> Create a BFD by calling the internal routine
<code>_bfd_new_bfd</code>, then call <code>bfd_find_target</code> with the
target string supplied to <code>bfd_openr</code> and the new BFD pointer.

</li><li> If a null target string was provided to <code>bfd_find_target</code>,
look up the environment variable <code>GNUTARGET</code> and use
that as the target string.

</li><li> If the target string is still <code>NULL</code>, or the target string is
<code>default</code>, then use the first item in the target vector
as the target type, and set <code>target_defaulted</code> in the BFD to
cause <code>bfd_check_format</code> to loop through all the targets.
See <a href="bfd_005ftarget.html#bfd_005ftarget">bfd_target</a>.  See <a href="Formats.html#Formats">Formats</a>.

</li><li> Otherwise, inspect the elements in the target vector
one by one, until a match on target name is found. When found,
use it.

</li><li> Otherwise return the error <code>bfd_error_invalid_target</code> to
<code>bfd_openr</code>.

</li><li> <code>bfd_openr</code> attempts to open the file using
<code>bfd_open_file</code>, and returns the BFD.
</li></ul>
<p>Once the BFD has been opened and the target selected, the file
format may be determined. This is done by calling
<code>bfd_check_format</code> on the BFD with a suggested format.
If <code>target_defaulted</code> has been set, each possible target
type is tried to see if it recognizes the specified format.
<code>bfd_check_format</code> returns <code>TRUE</code> when the caller guesses right.
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